DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 347 



is the anlage of the ductus choledochus. The pars hepatica by this time has 

 enlarged considerably and remains attached to the ductus choledochus by a 

 short cord of cells, the anlage of the hepatic duct. The pars cystica has also 

 become larger, its distal portion being somewhat dilated, and is connected with 

 the ductus choledochus by the anlage of the cystic- duct (Figs. 312 and 313). 

 The pars cystica grows into the ventral mesentery and thus becomes sur- 

 rounded by mesodermal tissue. The proximal portion continues to elongate to 

 form the cystic duct and the distal portion becomes larger and more dilated to 

 form the gall bladder. 



D.ch. 



.Du. 



V. pan. 



Fig. 311. — From a model of the duodenum and the primary evaginations of the 



liver and pancreas in a 5 mm. sheep embryo. Stoss. 



D.pan., Dorsal pancreas; Du.. duodenum; D. ch., ductus choledochus; G. bl., gall 



bladder; H. du., hepatic duct. 



The pars hepatica, or anlage of the liver proper, also grows into the ventral 

 mesentery, thus becoming surrounded by mesodermal tissue. As stated in 

 connection with the development of the diaphragm, the portion of the mesen- 

 tery into which the liver grows is involved in the formation of the septum 

 transversum (p. 376). Thus the developing liver becomes enclosed in the 

 septum (Fig. 330) . The mesodermal tissue gives rise to the fibrous capsule of 

 Glisson and to the small amount of connective tissue within the gland. 



Although the liver develops as a series of outgrowths from the original 

 evagination, there are certain features in its development which distinguish it 

 from glands in general. The outgrowths come in contact with the omphalomes- 

 enteric veins which are situated in the ventral mesentery (p. 263). They push 

 their way into and through the veins, breaking them up into smaller channels 

 (Fig. 310). They anastomose freely with one another, and the veins send off 



